The summer sky is full of starless voids, almost looking like holes in the universe. For much of the 19th and early 20th centuries, astronomers were at a loss to explain what they were. Were they actually empty regions or were they some sort of dark material that simply blocked the light from stars that lay beyond?

The first astronomer to make an in-depth study of these mysterious regions was Edward Emerson Barnard. Barnard's 1919 paper in the Astrophysical Journal, On the Dark Markings of the Sky with a Catalogue of 182 such Objects, outlined the results of his initial investigation, that these voids were anything but empty. Rather, they were vast clouds of opaque interstellar matter. Barnard's research was ongoing at the time of his death in 1923, but ultimately led to a second, expanded listing of "A Photographic Atlas of Selected Regions of the Milky Way" being released posthumously in 1927. This second publication expanded Barnard's catalog of dark nebulae to 349 total entries. A century later, his research still stands today as the benchmark for all subsequent investigations.

Whether it is due to light pollution, a lack of good charts, or maybe because there are so many bright objects to observe, many amateurs tend to shy away from seeking out dark nebulae. In a way, that makes sense. No other deep-sky genre so easily falls victim to less-than-perfect sky conditions than dark nebulae. The slightest haze or light pollution is usually enough to increase the sky's ambient background brightness enough that it swallows up these dark, cold clouds of interstellar dust grains.

This month's dark challenge is no exception. Barnard 86 (B86) is impossible to see in an urban sky since there is no light-pollution filter made to counter the destructive effects that our over-illuminated civilization has had on its visibility. Even a decent suburban setting could be bright enough to render B86 invisible. Head out to a dark, rural location, and you’ll have no trouble spotting this inky black cloud using 70-mm or larger binoculars, or a telescope with an aperture of at least 3 inches (7.6 cm).

Barnard 86 enjoys several distinct advantages over some other dark nebulae when it comes to visibility. For one, it lies sandwiched between the open cluster NGC 6520 and the orangish 7th-magnitude star SAO 186161. Its position in between these two bright targets makes zeroing in on its exact location a fairly simple exercise.  Another plus is that, unlike many dark nebulae that show only vague outlines, B86 has sharply defined borders.  We can see exactly where it starts and where it ends.  Finally, its rating of 5 on a 1-to-5 opacity scale indicates this cloud is truly dark.

Since Barnard 86 measures only 5' across, a magnification of between 100x and perhaps 125x will serve up the best view of it and its two bright neighbors. B86 appears decidedly wedge-shaped through amateur telescopes, with its western border facing SAO 186161 perhaps twice as long as the side closest to the cluster. NGC 6520 also spans about 5', with some two dozen stars visible through my 8-inch (20-cm) reflector.

Their close proximity in the sky naturally leads to the question of whether Barnard 86 and NGC 6520 are physically related or simply aligned by chance along the same line of sight. Astronomical evidence suggests a genuine spatial association. Both objects lie in the direction of the Galactic bulge, and distance estimates for each place them at approximately 6,000 light-years from Earth, making a physical relationship plausible.

Barnard 86 is likely part of the same molecular cloud complex from which NGC 6520 formed. Open clusters like NGC 6520 are born from such dense clouds, and it’s probable that Barnard 86 represents an as-yet uncollapsed remnant of the original stellar nursery. The stars in NGC 6520 are relatively young—just a few hundred million years old—so the survival of a dense, dark cloud nearby is consistent with the typical timescales of molecular cloud dispersal.

Thus, Barnard 86 and NGC 6520 are not merely an optical pairing; they are likely kin, sharing a common origin within the same star-forming region. Today, the brilliant stars of NGC 6520 light up the surrounding space, while Barnard 86 remains a dark, brooding sibling. Together, they make a wonderful study in contrast.

Incidentally, for those who might be traveling to Stellafane in Springfield, VT, at the end of this month, you will find that both B86 and NGC 6520 are on my 2025 Binocular Observing Olympics (BOO) list. I hope you'll try to see all 20 objects on this year's list and earn a special pin for the accomplishment. You can see and download the full BOO list as well as much more about the convention on Stellafane's website.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my website or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase.  Game on!